Since august 1993 the FIFA releases its World Ranking every month. Every nation is ranked based on their game results of the past four years with the most successful teams being ranked highest. Points can be earned in official matches against other FIFA members, where recent results have a higher weight.
The average ranking is calculated in the following way: First all the monthly rankings of a nation are added up and afterwards the outcome will be divided by the number of months. Example: All the rankings of Angola are added up -> 93th (in aug ’93) + 97th (in sep ’93) + 97th (in oct ’93) etc.. The answer of the sum is 17160. This number will be divided by the number of months. As of now (13-dec-13) the list was published 231 times. So 17160/231=74,29. This is the average rating of Angola.